Wireless And Digital Solutions For Smart Buildings

Designing a DAS For Train Stations

Electromag has delivered Distributed Antenna System (DAS) designs for over 13 train stations across the US and Australia. The design challenges we have faced varies from adding newer technologies to a site, extending coverage of existing DAS to more areas, upgrading the DAS (SISO to MIMO) to designing a completely new DAS (greenfield).

Capacity planning is a critical part of any DAS design for a train station. The population is very transient but the numbers can be estimated at peak hours for a worst-case crowding and used as a starting point. If the budget is slim, you may not be able to over-engineering a DAS for a future capacity requirement. A smarter way to approach capacity planning, is designing for the current worst-case capacity requirement, and in the implementation of the cable runs and port usage, allowing for easy future sectorisation.

Once we have a grasp on the capacity requirements, the next part of the requirement gathering exercise would be to answer the following questions (This is not an exhaustive list)

What are the acceptable design criteria? (For example: a coverage of 95% of the area with a minimum signal strength of -85dBm)

What will the technologies required for the station? For example: GSM, CDMA, UMTS, LTE etc (This will be determined by the operators or the system integrators)

Is there a tunnel? If so, how long is it? This information is critical for planning the ideal location for the Head-End equipment placement, mode of RF signal delivery in the tunnels, power requirements etc.

What are the tunnel dimensions and overhead clearances? This would help decide on whether to use antennas or say, chose a leaky feeder cable for signal propagation.

Where can the equipment room be located? Are there suitable IDF/MDF rooms already available? The head end-equipment requires space. The ideal Equipment room location would have AC power access, and would be optimally located to have cable runs that don't cost significant power losses.

Is there existing cable trays and cable ladders? Where are they located? This helps to plan cable routing for the DAS.

There are at least 3 different areas to design within a typical Train Station

The concourse

The platforms; and

The tunnels

Each of these areas has different considerations to take into account, and hence the approach may vary depending on the station. Each area is designed in iBwave or available prediction matrices. In iBwave, a propagation prediction can be used to predict the effectiveness of the DAS.

One consideration while designing for Platform Levels is to know that the DAS will not only cater for the passengers waiting on the platform but also to the passengers on-board the incoming or outgoing train. This means that it is important to take into consideration the Train Loss, which is the signal losses when penetrating to the inside of the train. This is typically assumed to be around 15dBm.

The tunnels, as mentioned above, are typically covered using a Leaky Feeder cable. The cable is "leaky" in that it has gaps or slots in its outer conductor to allow the radio signal to leak into or out of the cable along its entire length. Because of this leakage of signal, line amplifiers are required to be inserted at regular intervals, typically every 350 to 500 metres (380 to 550 yards), to boost the signal back up to acceptable levels.

An additional note on the design for tunnels - a strategy sometimes used to distribute power and manage hand-over of RF signal within tunnels, is to provide coverage via leaky feeders (or radiating cables) up to the midpoint of the tunnels before and after the station in consideration. This is done when the tunnels are quite long and cant be effectively covered by one DAS system. To illustrate this, let’s look at an example. Assume that Station A, Station B, and Station C are consecutive stops. A radiating cable from Station B would cover the tunnel from the midpoint of Station A and B to the midpoint of station B and C.

There are a several other considerations while designing for a Train Station depending on the need, and the effort and budget being allocated. For instance, recently we worked on a DAS design for one of the stations on the in Australia, which was a SISO to MIMO upgrade. Although MIMO is more complex in its design, it was a move in the right direction. Some statistics indicate that 90% of the traffic in indoor networks is DATA (only 10% is voice). A MIMO works better for LTE technologies to accommodate faster data rates and minimise signal degradation.

For help with assistance on you next DAS design, feel free to send us a question at This email address is being protected from spambots. You need JavaScript enabled to view it./">This email address is being protected from spambots. You need JavaScript enabled to view it. and we will happy to chat with you.

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